1. Field of the Invention
The present invention relates to a superconducting magnet and a magnetic resonance imaging (MRI) apparatus using the same, and particularly to a superconducting magnet for an open type of magnetic resonance imaging apparatus and an MRI apparatus using the same.
2. Description of the Related Art
An MRI (Magnetic Resonance Imaging) apparatus including a superconducting magnet for generating a tomographic image of a subject on the basis of hydrogen nuclear density by measuring electromagnetic waves emitted by hydrogen nuclear spin based on the nuclear magnetic resonance (NMR) phenomenon is known.
Because an electromagnetic field generated by an electromagnetic wave emitted by a hydrogen nuclear spin is proportional in intensity to a static magnetic field in the measurement region, it is necessary to increase an intensity of the static magnetic field to improve a resolution of the tomographic image. To generate a high intensity static magnetic field, a superconducting magnet is used. Further, it is necessary to increase homogeneousness in magnetic field in the measurement region to obtain a high image quality and high resolution without distortion in the tomographic image. Accordingly, a magnetic member may be disposed in the superconducting magnet. Further, an MRI apparatus using a magnetic member for improving homogeneousness in magnetic field intensity in a measurement region to suppress distortion in the tomographic image is known. In the MRI apparatus, a homogeneous magnetic field with a high intensity and a high homogeneousness in the static magnetic field (magnetic flux density) can be generated in the measurement region.
Generally, the magnetic member is disposed in the atmosphere at a room temperature. On the other hand, JP 2001-224571 A and JP 10-97917 A disclose MRI apparatuses including such a magnetic member disposed in a coil vessel filled with liquid helium having a cryogenic temperature for cooling superconducting coils. Further, JP 2005-144132 A discloses an MRI apparatus including such a magnetic member in a thermal shield at a middle low temperature for covering a coil vessel having a cryogenic temperature to shield the coil vessel from a heat radiated from the vacuum vessel.